static IP to dhcp :o

Hi all, just a small question...can you tell me how can i change the original IP settings from the 192.168.0.100 static address to a DHCP generated IP ( is a public address however is dhcp generated from my ISP, is the same IP ever.)
I have a fresh Ubuntu 5.10 installation working very fine whit the static IP.
I need to put this server online as possible and i need to know if theres a way to do that.